1. Field of the Invention
This invention generally relates to a method for providing an optical instrument and, in particular, relates to a method for reducing sidelobe impact of low order aberration disk in a coronagragh.
2. Description of Prior Art
A coronagraph is an instrument originally invented to observe the Sun's corona at times other than a solar eclipse. Its action is to reduce the diffracted sidelobes of the bright solar image. It can also be used to observe unresolved sources and has found application in planned detections of extrasolar planetary systems. Planets in orbit around other stars would appear as very faint sources in orbit around the very bright central stellar source. For a solar system similar to our, own the planets would be of order a billion times fainter than their parent star. The image of the bright parent star when viewed in a telescope (in fact the image of any unresolved source) under diffraction limited conditions is surrounded by a halo consisting of diffracted light from the parent star. For the nearest stars, this halo is the dominant source of background against which the much rainier planet must be imaged. For nearby stars and planets similar to our own this background could be 100,000 times brighter than the planet. In this situation, the coronagraph is employed to reduce the diffraction sidelobes of the parent star and place the planet in a more favorable brightness balance. All techniques which improve this balance aid in the detection of the planet.
A classical coronagraph 2 is shown in FIG. 1 in a demonstration breadboard. The laser and spatial filter simulate a star and the plano-convex objective simulates the aperture of the telescope observing the star. The coronagraph instrument begins at the opaque occulting mask (10) at the first focus of the system. The lens (16) reimages the entrance aperture (12) which, because of the occulting mask, now shows a concentration of diffracted light around the edge of the aperture image. This diffracted light is removed by a stop, called a Lyot stop (14), which is undersized with respect to the aperture image. The beam is then brought to a second focus at which a recording device (camera or CCD) is located. For a source occulted by the mask there is a reduction in the diffracted light halo whose magnitude depends on how much of the original image was occulted, and on how much the Lyot stop was undersized with respect to the entrance aperture. Any source which is not behind the occulting mask is essentially unaffected by the coronoagraph. Thus, for the planet detection problem, the occulting mask size is limited by the need to see the planet, while still occulting its parent star. The need to undersize the Lyot stop relative to the entrance aperture of the system means that some planet fight must be lost to the stop.
The relationship between diffraction reduction, occulting mask size, and Lyot stop size is a complicated, non-linear function which can be determined for specific parameters with a numerical model. Such a performance curve is shown for a classical coronagraph as the top two curves in FIG. 2 which give the effective diffraction reduction both near the edge of the occulting mask (top-most curve), and 2.5 mask radii away from the mask edge (bottom-most of the two curves) as a function of the Lyot stop diameter scaled to the entrance aperture (also called the entrance pupil). Thus a Lyot stop diameter of 0.85 means the stop diameter was 85% of the diameter of the entrance aperture. In this case the occulting mask covered the first five diffraction rings of the diffraction pattern of the central star. This corresponds to about a half arcsecond for a meter class telescope observing a star in visible fight. This is the kind of dimension typically needed in the planetary detection problem. It is evident from the behavior of the curve that diffraction reduction of no more than a factor of one hundred can be realized by a classical coronagraph for this range of parameters. Since the occulting mask is already of the order of the dimension needed for planetary detection, and since adjusting the Lyot stop produces no appreciable improvement this is approximately the limit of performance of a classical coronagraph in the planetary detection situation.
The actual coronagraphic instrument is made up of the elements between the occulting mask 10 and the camera 22 as shown on FIG. 1. In operation, the laser 9 and spatial filter 11 are used in combination to simulate a distant star. The lens 8 and pupil 12 together simulate the telescope. Thus, the entire breadboard simulates a coronagraph on a telescope looking at a star. The beam splitters 17, 19 are mirrors that can be added or removed from the setup. When the first mirror 17 is in place, it feeds the microscope 21 and is used for alignment of the occulting mask 10 or inspection of the pupil 12. When it is not in place, the coronagraph operates normally. The second mirror 19 is added or removed depending on whether one wants to feed the film camera or TV camera.
It is known that the performance of the coronagraph can be improved by apodizing, or shaping the transmission profile of the occulting mask so that its amplitude transmission follows the profile: EQU T(r)=1-e.sup.-(r/.sigma.)
where .tau. is the radial coordinate in the focal plane and .sigma. is a free parameter to be determined. This apodization works because it the produces a greater concentration of diffracted energy in the reimaged entrance aperture rendering the application of the Lyot stop more effective. This is shown by the bottom set of curves in FIG. 2 which gives the diffraction attenuation at comparable positions to the set of curves above for an apodized occulting mask having a fifty percent intensity transmission at the same focal plane radius as the edge of the opaque occulting mask in the example above. Note that with the apodized mask, as the Lyot stop is decreased in diameter, there is a dramatic reduction in diffracted light approaching four orders of magnitude. Even with this improvement in performance it is important to note that in a real optical system, even in a space-borne environment free from the scatter effects of the Earth's atmosphere, the halo of light surrounding an unresolved source will also consist of scatter from the fabrication errors of the optical system and the sidelobe effects of the system alignment errors. These must be reduced or controlled to the same level as the reduced diffraction sidelobes in order to render the coronagraph action useful.